Radio frequency tube with low internal impedance



July 6, 1954 c, SCULLIN 2,683,237

RADIO FREQUENCY TUBE WITH LOW INTERNAL IMPEDANCE Filed Dec. 29 1951 I T"? :2 life i \l I I IIIIIIIIIIIII V line I v 5 I Z 1 13 g INVENTOR C. HJS'C'UZZ/N.

AITTORNEY Patented July 6, 1954 RADIO FREQUENCY TUBE WITH LOW INTERNAL IMPEDANCE Carl H. Scullin, Florham Park, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 29, 1951 Serial No. 264,027

3 Claims.

' My invention relates to electrical discharge tubes and in particular relates to a novel controlelectrode arrangement for electrical discharge tubes of the high-vacuum type. It is particularly useful for high power oscillation generators or amplifiers of higher radio frequencies.

For many fields of radio frequency power service, it is desirable to have a generator or amplifier of low internal impedance; that is to say, an electronic tube capable of operating at a relatively low plate voltage and high plate current. Such a. low impedance source is much easier to match with the load impedanoes ordinarily encountered in both radio broadcasting and radio frequency heating than are high impedance sources. A low impedance tube likewise makes it possible to use a cheaper and simpler modulation transformer, and facilitates the attainment of wide bandwidth in television service.

One object of my invention is to provide agridcontrolled electron tube which shall have a low anode-to-cathode impedance.

Another object of my invention is to provide a novel type of grid-controlled electron tube.

Still another object of my invention is to provide an electrical discharge tube with a novel type of control electrode.

Yet another object is to provide a novel type of amplifier.

A further object is to provide a grid-controlled tube in which an unusually small fraction of the electron-emission of the cathode finds its way to the control-electrode during the time of electron conduction to the anode.

A still further object is to provide a grid-controlled electrical discharge tube in which the control-voltage needed for effective operation is unusually low. 7

Yet another object is to provide a grid-controlled electron tube in which the ratio of anodeto-cathode impedance to anode-to-cathode capacitance is lower than in conventional tubes of the present art.

. A still further object is to provide an electron tube having a control electrode in which secondary emission from the electrodes is minimized.

Other objects of my invention will become apparent to those skilled in the art upon reading the following specification taken in connection with the drawings, in which:

Fig. l is a view in sectional elevation of an electron tube embodying my invention, taken on line I-I of Fig. 2.

Fig. 2 is a view in section along the plane 11-11 of Fig. 1.

Fig. 3 is a graph showing curves of voltage and current under one condition of use of a tube embodying the invention.

Referring in detail to Fig. 1, an electron tube.

embodying my invention comprises a vacuumtight container I having a. cylindrical metallic wall-portion 2 which forms its anode, and a glass base-portion 3 sealed thereto. An annular ring 4 of material, such as described in Patent No. 2,062,335 sold under the trade-mark Kovar, is brazed to the lower edge of the anode 2, and an annular component 5 of the glass base-portion 3 is sealed to ring 4. A second annular Kovar ring 6 has the lower edge of the glass member 5 and the upper edge of a glass bowl 1 sealed to it. The base-portion of the bowl I has a Kovar ring 8 sealed through it, and also a center-post 9 which projects along the axis of anode 2 nearly to the closed upper end thereof.

The center-post 9 has fastened to it an insulating disc I I provided With a downward-projecting hub. On the upper face of disc I I is supported a metal disc l2 from equally-spaced points on the periphery of which there project a plurality of fingers l3. Adjacent the lower face of disc H is a metal disc M which is centered about centerpost 9 by the insulating hub of disc I I, and which is electrically connected to ring 8 by members l5. Half way between posts I3 there project upward a set of posts l6 which pass through insulating openings in disc [2, and through disc II to anc'hor in disc I4. A suitable spring-member ll slidably surrounds center-post 9 and its upper end biases upward a sleeve I8 which is slidable on center-post 9. The upper end of sleeve l8 engages an insulating collar is; which slidably engages the upper portion of center-post 9, and which supports a plate 2 I.

The ring 6 is provided with three projecting arms 22 at degree intervals which support a control-electrode 23 having the form of a cylinder coaxial with the anode 2.

As is shown more clearly in Fig. 2, the control electrode 23 is a tube of high-melting metal, such as molybdenum, having longitudinal grooves 24, one for each vertical strand 25 0f the cathode filament. The grooves 24 are preferably spaced away from the filament 25 by the minimum distance which will withstand the potential during the positive phase of voltage impressed between the control-electrode and cathode, and which will provide sufiicient mechanical clearance during operation of the tube, allowing also for expansion of the cathode when heated to its operating temperature. To give a specific example, the tube may have an .018" diameter filament wire with a clearance between filament wire and control electrode slot of .005" on each side. This geometry will result in an estimated control electrode current density of 4 amp/cm. at a positive instantaneous control voltage of about 40 volts. This current density is greater than canbe supplied by. most practicalcathodes; thus the control electrode would be successfully robbing the anode of virtually all current. The anode 2 may;

be spaced about from the surface of the control electrode 23, its minimum instantaneous ..p0,e.

tential, at the time of maximum anode current flow, being about 800 volts positive whereit would draw about 500 ma. cm? fromnthecathoder-z' The control electrode would be negative'at this =in-' I stant and thus draw no current.

The strands 25 of the cathode are all supported at their upper ends from posts 2-6;projecting downward from the disc 2|. The lower ends ofalternate filaments 25=are respectively attached to the fingers [3 set in disc 12 and fingers [6 set in disc [4 as already described.-: A heating-cum rent supply for the cathode is connected between cylinder 8 and center-post'fl. Thus heating current for the cathode flows from center-post 9 into disc [2, thence up fingers l3 to, -let us.say,- the odd numbered filaments 25 and discZ I, then down the even-numbered filaments 25-to fingers IG'and disc [4, and member IE to cylinder 8. Thespring I! exerting upward bias'ondisc 2I holds all-the filaments 25 taught and straight in the grooves 24.

Withan electrode structure such as I have described and an anode potentialof over 1000 volts'positive to the cathode, a positivecontrol electrode voltage of 40 volts willcausenearly all ofthe electrons emitted by the cathode strand 25 to strike the controlelectrodeZiS, thus efiEectively cutting off all current flow from'the cathode to .the anode. Nevertheless,'the.small spacing .of the control-electrode from the cathode makes a relatively small control voltage eifective and-the power input to the control-electrodeis still relatively small.

Reduction of the positive control-electrode voltage. to zero or a small negativevaluesuifices however to cause substantially all the electrons emitted by the cathode to flow to the-anode. Since there is no control-electrode structure actually positioned between the cathode strand and the anode, the current flow between the two approaches the conditions present in. a diode, and the voltage drop from anode to cathode remains'low, thus giving a tube of high perveance, i. e., a low anode-to-cathode resistance.

Such a tube is particularly advantageous for use in a class C amplifier, wherethe controlelectrode is operated with a positive bias. Fig. 3 indicates the operation under such conditions, curve e representing the anode voltage alternating about a D. C. plate-bias Eb. The controlelectrode voltage 6g alternates about a positive grid-bias EC and, unlike conventional class C amplifier operation, in phase with the plate voltage e The curve i shows-the plate current, which flows during the negative half-cycle 'of the plate alternating voltage.

I claim:

1. :Anzelectrical'discharge device comprising a cylindrical anode, a cathode constituted by a pluralitywof' strands 'in a circular series coaxial with saidcylindri'cal anode and inward therefrom, and-a control electrode having portions thereof interposed between said strands whereby said electrode provides a plurality of parallel grooves each containing one of said strands, and each of said strands having a clearance from the said electrode portions that formthe groove therefor less than-the diameter of the strand.

2. vAn electrical-discharge device comprising a cylindrical anode,. a cathode constituted by a plurality of strands in a circular seriescoaxial with said cylindrical anode and inward therefrom, and a control electrode having radialportions thereof interposed between .said strands whereby said electrode provides-a plurality of panallel grooves each containing .one of said strands, and each of said strands having a diametermore than'half of the widthof the groove containing the. strand.

3. An electrical discharge device comprising a cylindrical anode, a cathode constituted -by a plurality of ,strandsina circular. series coaxial with and inward of 'said cylindrical'anode, and a coaxial control electrode having a plurality of parallel grooves therein parallel to the axis of said anode and electrode, each groove. having a strand of said circular series of strands therein, and said groove having a width greater than the diameter of the strandtherein andaffording a clearancebetweenthe-strand and each-side .of the groove inthe .range betweenonethird and one... quarter .of the .strand diameter.-

References .Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,751,418 Paul Mar. 18, 1930 2,022,988 Thompson Dec; 3, 1935 2,248,712 Litton July 8, 1941 2,544,664 Garner et a1. Mar. 13, 1951 FOREIGN PATENTS Number Country Date 327,634 Germany Nov. 2, 1920 

